U.S. Pat. No. 4,598,850, the disclosure of which is hereby incorporated by reference, as well as German Patent 35 05 515 (to which European EP 192 146 corresponds), all assigned to the assignee of the present application, describe a system in which a pull-in element is driven by a transport wheel which is located for engagement with the pull-in element. In accordance with the disclosure of the aforementioned German Patent 35 05 515, the spacing between the transport wheel and the pull-in element is so selected that the transport wheel extends into the path of the pull-in element. The transport wheel is formed as a sprocket or gear wheel, and the pull-in element is formed either as a chain or has a worm-like pull-in element or pull-on arrangement, to provide for interengaging power transfer engagement between the wheel and the pull-in and pull-on element. A resiliently biassed ball is located below the transport path, extending into the path of the pull-in element which, when the pull-in element is fed into the transport path, is deflected to operate a motor to start driving the pull-in element. Additionally, the ball engages the pull-in element and presses it upwardly to provide for positive interengagement between the teeth of the transport sprocket wheel and the chain gaps or spiral or worm projections on the pull-on element, in dependence on the construction thereof.
The ball is subject to wear and the projecting portions of the pull-on element likewise are subject to wear. When threading paper into a printing machine, it is important that the threading operation is carried out smoothly to prevent sudden jolts being applied to the paper, which might cause the paper to tear. If, due to a fortuitous position of the pull-on element, a gear tooth of the transport wheel happens to meet a projection or chain link of the pull-in portion, jolts have occurred.